I have an "FM oscillator 2W (88... 108 Mhz)" kit from "GENIUS ELECTRONICS"
I have some questions about it. First, may you make a look on the circuit and give me your view if it will work fine or what?
I compared the connections in the circuit and the real connections on the PCB (as shown in the PCB layout in the picture), I found a difference.. a mistake in the connections. The connection mistake is more probably to be with the circuit diagram and not with the PCB since the kit is being in the market from a long time.
Here is the problem: According to the circuit diagram, the positive terminal of the battery is connected to the emitter of T2 and to C5... while on the pcb it is connected to the second side of R5, it is connected to the emitter of T1 and C4, R4, and the +ve terminal of the battery is connected to one of the terminals of the mic.

This is the most important thing to know for now but I want to ask two other questions:
L1 is printed on the pcb as shown in the picture, T2=sf118 have a metal case as bc109/bc107 but its size is about 3 times that of bc109 (I didn't found its datasheet). Do you think this fm 'oscillator' is really 2W? if it's 2W its range should be 3KM or even more right? There is no supplied information about the antenna.... what do you think?

Point 'A' (on the pcb) it connected to ground, what do you think it is?

Be aware the transistors are both PNP types.
I do not know the SF118.
Point A is the connection for the antenna.
A wire of about 75 cm will do well as antenna.

Greetings,
Bertus

Click to expand...

Yes it's obvious that they are PNP.. maybe I don't know how this affects the connections (+ connected to mic?...)
Ok but which connection is the right one? that of the circuit diagram or the PCB? and is it ok for the antenna to be connected from the ground?

The antenna only must be connected to this point A.
When you want the antenna away form the PCB then you can use a 75 Ohm coax.
The core is connected to point A the sleeve is connected to the ground.
You can make a dipole antenna using two wires of 75 cm on the coax.

bertus, I can understand what you are saying well, A (antenna) should be that point at the collector of T2 and VC and L right? Please look and examine where point A is connected on the PCB. It is obviously connected directly with the ground.

As the power is generated by the oscillator itself, I do not expect much of the range.
It will be larger if the power supply is higher (more power).
The stability will also be not so good as heating of the T2 will give a drift.

Thank you very much bertus. I bought this kit to use in a project I'm building. It is labeled 2W and 2W fm transmitter fits my need. Now I noticed that this kit can't transmit a clear signal for a distance of about 2KM.. I want a simple circuit to do this job. I got this one:

This is a well designed basic FM transmitter that you can easily recive the signals transmitted from this transmitter in a 1-2km range with using a normal FM reciever. Another property of this circuit is that the bobin is placed on the printed circuit board. The input sounds amplitude can be adjusted by using the P1 potentiometer.

For this circuit http://english.cxem.net/radiomic/bug22.php
If I replaced T2=2N2219 with 2N3553 (65V, 1A, PQ=2,5W(175MHz)), I think the transmitting range will increase much more and so I solve my problem. What do you think?

I confess that RF defeated me . Actually, I tried a lot to build a powerful transmitters earlier, my theory background was insignificant, but now it's better.. I still have problems with rf circuits even though I love it. One of the problems is that I hate working with coils and I find it hard many times. The main problem is with the parts needed, wires for coils with different thicknesses and-ferrite core-rf transistors...etc because I live in a remote village far from electronic components stores...etc

I will try the last simple circuit I provided and another one, I'll try to edit them and see...